I have laid lots of tile and I would suggest instead of mortar, use a “tile adhesive” , it sticks tile down better and dries faster, it also works better than mortar when used near water.

As far as grout, use an “unsanded” grout

I would use a acrylic fortified thin-set for your adhesive. . .

And a non sanded grout using very small grout lines perhaps a 16th of an inch, I usually just use shim spacers for this. . .

You can buy a sealer that is just for stone and all of these can be purchased at Home Depot. . . However a sealer will not prevent your marble from being etched by anything containing acid. . .

So you will not want anything containing acid to contact your counter or it will instantly remove the polish, wine, citrus, tomatoes etc. . .

yes tahoeman is right. thinset mortar, don’t use a mastic adhesive. unsanded grout, less than 1/16″ space. the only thing I can add is use 3/4″ plywood preferably BC, and then 1/2″ backerboard as your substrate. you say know the cons, but I’d reiterate, it will scratch easily and can break easily, so try to get 100% coverage on the stone. I back butter natural stones. And seal the whole tile.

I’m sure it will be beautiful, just handle it with a lot of care to maintain that beauty.
Good Luck

Thousands of sq. Ft. Of tiling later, I suggest I disagree with number one no offense meant.

I’ve done shower stalls and schoolway halls, etc.

And Quickset is as good a mortar as anything.

It’s a proven, long standing mortar.

You NEED to remove any laminate or surface covering before beginning,,,,please.

I disagree again with number one,,, No offense, but non sanded grout is best used in grout lines larger than 1/4 inch, and hopefully you have planned 1/4 inch or less for a kitchen counter top.

A quality, polymer bases, sanded grout will be just fine.

Sealing the grout will help, but will eventually wear off any tile it gets on. I have no idea of the color, but suggest not white grout.

Will you have bullnose tile for the facing edge? For the back splash?

You have the proper sponges? and will wipe and wet more than once? BTW the flat blade knife with 1/4 inch teeth is what I use on floors.

You might seek out one with 3/16th teeth for a counter?

Rev. Steven

When we tiled out kitchen counters, we screwed cement backer board down over the laminate.

It worked just fine.

We were going to replace the sink and cook top so the extra thickness was not a problem.

Im sorry Ever Wolf, i’m sure you’re a good contractor, but you’re no tile installer. Non-sanded grout is for grout joints 1/8″ or smaller. Sanded grout is for grout joints 1/8″ or larger.

The sand is for stability in larger grout joints. NEVER use sanded grout in anything smaller than 1/8″
When installing marble tile, it is customary to butt the tiles against each other with no grout joint at all.

The edges of the tiles are machined with a micro-bevel so there will be a slight groove between them that are to be filled with an unsanded grout. Use latex fortefied WHITE thinset mortar. Marble can be a bit translucent and the darker grey thinset can show through.
There is alot more to consider such as what to do with the edges, but that is too involved to try to explain
good luck with your project!

JC 1129 is correct and there are many more variables to consider but as you stated, the tiles were too good to not use and were free so the only thing you’ll possibly be out of is time.

As for the edge treatment you might want to consider a hardwood such as maple or oak with a polyurethane seal and perhaps a bevel or ogee w/fillet routed edge.